1. Field of the Invention
The present invention relates generally to the field of medical implants. More specifically, the present invention relates to an apparatus for providing a signal representing the status of a sensor in a medical implant.
2. Description of the Prior Art
Ever since the introduction of rate responsive implanted cardiac stimulators, a number of different parameters have been used for determining the activity level of the patient, which in turn is used for controlling the rate at which the heart of a patient is to be stimulated by the pacemaker. One of the most common sensors used is the piezoelectric accelerometer.
Another form of sensor is the intracardiac piezoelectric pressure sensor.
Unlike the piezoresistive and piezocapacitive sensors piezoelectric sensors are not energy consuming, on the contrary they generate their energy themselves. Piezoelectric sensors are also arranged to alter the mechanical stress of the piezoelectric material in response to a change of loads emanating from for instance an acceleration of a seismic mass or from a change in pressure acting on the sensor. This results in a transport of electrons or electrical charges within the material, which provides a change in voltage across the piezoelectric sensor. This voltage corresponds to the load to which the sensor is subjected.
A problem related to measuring the voltage across a piezoelectric sensor is the leakage of charges that occurs, negatively affecting the accuracy of the measurements. In an attempt to solve this problem, use has been made of a voltage amplifier having a very high input impedance. This requires, however, a very large resistance component, which is undesired within a medical implant. Furthermore, the problem related to leaking charges is still not completely eliminated, and the use of a memory function of some sort would be required. The problem of leaking charges is of particular interest when the piezoelectric sensor is subjected to relatively small changes in load over long time periods, such as small changes of pressure over a long time or changes in posture.
An object of the present invention is to provide a method and apparatus for determining the status of a piezoelectric sensor that takes the leakage of charges mentioned above into account.
A further object of the present invention is to improve the possibilities of evaluating the status of a piezoelectric sensor.
These objects are achieved in accordance with the present invention wherein status related sensor output changes are substantially continuously detected, and based thereupon a signal representing the actual status of a sensor is generated. Advantageously, said signal is generated by integration of the sensor output changes.
Preferably, use is made of a sensor of the type in which status changes generate changes regarding electrical charges in the sensor. Thus, the sensor is suitably of the piezoelectric type.
In accordance with a preferred aspect of the invention, positive and negative charges generated by the sensor, as more closely discussed below, are substantially continuously detected and removed from the sensor, thereby keeping the output voltage of the sensor at a substantially constant zero level, while at the same time providing an output current which can be the basis for an integration in order to produce the signal.
According to an embodiment of the invention, this can be accomplished by connecting the charge-producing sensor to a circuit having the characteristics of an input impedance that is extremely low or redundant. As a result, charges generated by the piezoelectric sensor will immediately leak to, or be collected or removed by, the connected circuit. This also means that there will be no problem with uncontrolled leakage of charges from the sensor, as is the case in the prior art.
As indicated above, a change of load generates a change of charges in a sensor of the piezoelectric type, all charges generated being collected, i.e. detected and removed from the piezoelectric sensor, by the connected circuit. A change of load can be either positive or negative. A positive change of load will generate an internal transport of charges in a direction opposite that caused by a negative change of load. Furthermore, if a transport of charges in one direction generates a positive voltage across the piezoelectric sensor, a transport of charges in the opposite direction generates a negative voltage. Hence, for restoring a zero voltage level across the sensor from a positive voltage level, there must be a transport of xe2x80x9cactualxe2x80x9d charges from the sensor, while for restoring a zero level from a negative voltage, there must be a transport of xe2x80x9cactualxe2x80x9d charges from the connected circuit to the sensor.
In accordance with the above, the actual charges supplied to the sensor for restoring a zero level will hereinafter be referred to as collected or removed negative charges, and the resulting current will be referred to as a negative current. Correspondingly, the actual charges removed from the sensor will be referred to as positive charges, and the resulting current as a positive current. Therefore, both the supply and the removal of charges to and from the sensor hereinafter will be referred to as a collection of charges, wherein a supply of charges to the sensor will be referred to as a collection of negative charges, and a removal of charges from the sensor will be referred to as a collection of positive charges.
The charges generated in a sensor of the type discussed correspond to the load (e.g. acceleration and/or gravitational force or pressure) to which the sensor is subjected. Accordingly, each generated charge represents a certain change of load. A greater change of load generates more charges; a more rapid change of load provides a more rapid generation of charges; and an change of load in one direction generates positive charges and an change of load in the opposite direction generates negative charges (in accordance with the above stated definition of positive and negative charges). Hence, the electric charges generated by the sensor per time unit, i.e. electric current, correspond to the amount and the direction of the change of load and, hence, to the time derivative of the load to which the sensor is subjected.
The charges generated by a piezoelectric sensor, as described above, are provided to a circuit for detecting and removing these charges. Since the number of generated charges per time unit, hereinafter referred to as the sensor current or sensor output current, is proportional to the time derivative of the change of load, an integration of said current will result in an integrated value or signal that is proportional to the load.
The circuit for receiving the current (detecting and removing the charges) is, according to the invention, arranged to integrate said current, i.e. to quantify and to cumulate the charges generated by the sensor. Accordingly, the resulting value from this integration will represent the net amount, i.e. considering the sign of the generated charges, of charges generated by the sensor. Thus, the integrated value, or signal, will be directly representative of the load to which the sensor currently is subjected. The integrated value can therefore be seen as a recreation of the voltage that would have existed in the sensor, provided that there would have been no leakage or deliberate removal of charges at all. Thus, the present invention solves the problem regarding obtaining an absolute value representative of the level of for instance a constant acceleration or gravitational force or pressure by the use of a piezoelectric type sensor.
As stated above, the restoring in the sensor of a zero level from a negative voltage level would require a supply of charges from the connected circuit to the sensor. According to an embodiment of the invention, the supply of charges can be provided by connecting a constant direct current, hereinafter referred to as a DC signal, to the sensor and the circuit. If the magnitude of the DC signal exceeds the possible maximum magnitude of the positive and the negative sensor current, the charges or the current supplied to the sensor for restoring the zero level will be provided by the added DC signal. As a result, the connected circuit will be provided with a combined signal, this combined signal being the sum of the DC signal and the sensor current. The combined signal will, e.g., have the magnitude of the DC signal when the sensor is not affected by a change in acceleration and/or gravitational force or pressure; a magnitude greater than the DC signal when the sensor is affected by a positive change in load, for instance acceleration and/or gravitational force; and a magnitude less than the DC signal when the sensor is affected by a negative change in said load.
As described above, the connected circuit integrates the sensor current. According to preferred embodiments of the invention, this integration can be accomplished by first subjecting the sensor current to a current to frequency conversion. The provision of an added DC signal to provide a combined signal, as described above, is particularly advantageous when used in conjunction with a current to frequency converter, in that the combined signal will always be kept positive and the frequency can be kept proportional to the level of the combined signal.
The current to frequency conversion produces a frequency signal that will be provided to a counted means for counting the pulses comprised in the frequency signal. The counting operation will generate the desired integrated value, after compensation for the contribution from the added DC signal, that will be directly representative of the actual acceleration or gravitational force by which the sensor is affected.
The contribution of the added DC signal, however, must be eliminated in order to obtain an integrated signal representing the immediate influence of the load on the sensor. According to an embodiment of the invention, the contribution of the added DC signal can be removed by deducting in the counter a counter value corresponding to the contribution from the DC signal. After each deduction, the counter value, i.e. the integrated value, will represent the contribution from the sensor current only, and, hence, from the load to which the sensor is affected.
The value to be deducted, herein referred to as a deduction value, can be obtained by disconnecting the sensor from the connected circuit for a given time period, and by registering the pulses in the frequency signal during said time period. Disconnection of the sensor can simply be provided by a switch. When this time period expires, the number of pulses registered during this time period is stored as the deduction value and the operation of the connected circuitry continues, using the updated deduction value, as described above. The operation for obtaining the deduction value can be performed at given time intervals, but is preferably performed when there is no sensor current.
In another embodiment of the present invention, the problem in compensating for the contribution of the added DC signal can be solved by providing two parallel signal paths, each path being provided with a separate DC signal, as described above, and including a current to frequency converter, a first switch for switching the sensor current between the two signal paths, a second switch for switching the respective frequency signal from the respective signal path between incrementing and decrementing inputs of an up-down counter, and an up-down counter.
The sensor current is periodically switched between the respective paths, so that the sensor current is half the time provided to the one path, half the time to the other path. As a result, the converted frequency signal output by each path will half the time comprise the converted combined signal, half the time a frequency conversion of the added DC signal. The converted signal, when including the contribution of the DC signal only, can be seen as an idle frequency signal. Obviously, when the sensor current is zero, a frequency conversion of the combined signal will have the same frequency as the idle frequency signal, regardless of the state of the first switch means.
The frequency signal output by each signal path is periodically switched between incrementation and decrementation inputs of an up-down counter. This switching is preferably performed in conjunction with the switching of the sensor current between the respective signal paths, so that the path presently receiving the sensor current is connected to the incrementing input of the up-down counter, and that the path presently not receiving the sensor current is connected to the decrementing input of the up-down counter. Hence, the respective frequency signal will increment the counter when including the contribution of the sensor current, and decrement the counter when not including the contribution of the sensor current. Accordingly, the contribution of the respective added DC signals will be completely eliminated and the integrated value output by the up-down counter will be directly representative of the current generated by the sensor. The contribution of the respective added DC signal will be completely eliminated, regardless of any drift of the DC signal over time and regardless of the difference between the DC signals.
According to this embodiment, the counter value, i.e. the integrated value, is constantly being updated and at all times represents the load to which the sensor presently is subjected.
One way of determining the activity level of a patient is to use a piezoelectric accelerometer in a medical implant to determine the physical activity of the patient and consequently the rate at which the heart of the patient is to be stimulated.
However, the heart rate in a healthy individual is also dependent of the individual""s static or long term physical body orientation or posture, or a change from one such orientation to another, e.g. from standing to lying down. The intrinsic heart rate is even dependent of whether the individual is lying in a supine, i.e. on his/her back, or in a prone position, i.e. on his/her face. Therefore, there is a need for establishing both the activity level and the body posture of a pacemaker patient, in order to control the operation of the pacemaker in dependence of the activity level and the posture of the patient.
A number of different methods and devices have been proposed for determining the physical orientation or posture of a patient. Generally, accelerometers are used for determining posture, as described for instance in European Application 0,845,240. This is due to the fact that gravitational force affects an object in the same manner as would a corresponding constant acceleration force. By determining the effect of gravitation on an accelerometer that is sensitive to acceleration forces in a certain direction only, the gravitation component in this direction can be measured and, hence, the angle between the axis of sensitivity and the direction of the gravitational force can be determined. Knowing the orientation of the accelerometer relative the patient, the posture of the patient then can be easily established.
The accelerometer also can be combined with one or more accelerometers having different directions of sensitivity, preferably perpendicular to that of the first accelerometer. Thereby, the possibility of detecting different postures of the patient will increase. For instance, the combination with an accelerometer having a sensitivity in the right-left direction of the patient, would enable distinguishing an upright position from a position where the patient is lying on his/her side.
Since the changes in acceleration and gravity connected with changes in posture are relatively slow compared to the changes in acceleration connected with normal physical activity and the device according to the invention takes the leakage of charges from the piezoelectric accelerometer into account, the invention is of particular interest in piezoelectric devices for detecting changes in posture.
As discussed above, the constantly updated integrated value represents the acceleration and/or gravitational force (i.e. the component of the gravitational force in the direction of sensitivity of the accelerometer) to which the accelerometer presently is subjected. The maximum contribution the accelerometer can be subjected to by the gravitational force corresponds to an acceleration of 1 g (9.81 m/s2). However, accelerations associated with heavy exercise, such as running, can significantly exceed 1 g, sometimes even exceed 2 g. Therefore, the integrated value will suitably be subjected to further processing in order, e.g., to distinguish between contribution from gravitation and contribution from physical activity.
According to an embodiment of the invention, the constantly updated integrated value can be provided as a digital output signal from the described counter to a posture evaluation unit for determining the posture of the patient. This posture evaluation unit, or circuitry connected between the posture evaluation unit and the counter, performs a digital low pass filtering of the integrated signal. This low pass filtering, preferably having a cut-off frequency of less than about 1 Hz, preferably about 0, 5 Hz, will effectively filter out the contributions of activity, heart beats etc. The low pass filtered integrated signal then can be compared to threshold values for obtaining a posture value indicating the actual posture of the patient. This posture value can then be provided to a control unit for controlling the operation of a pacemaker in accordance with the posture of the patient, in a known manner.
Likewise, according to a further embodiment of the invention, the integrated value also can be provided as a digital output signal to an activity evaluation unit for determining the physical activity of the patient. This activity evaluation unit, or circuitry connected between the activity evaluation unit and the counter, performs a digital band pass filtering of the integrated signal. This band pass filtering preferably has a lower cut-off frequency of about 1 Hz, and has a preferred upper cut-off frequency of about 10 Hz, preferably about 6 Hz. The band pass filtered integrated signal can then be evaluated in a known manner for obtaining an activity value indicating the physical activity of the patient. This activity value can then be provided to a control unit for controlling the operation of a pacemaker in accordance with the physical activity and the posture of the patient.
In another embodiment of the invention, use is made of a piezoelectric accelerometer formed by a two layer beam, one piezoelectric layer and one supporting layer, this beam being fixed to a mounting surface at one end and provided with a weight at the other end. Thus, when affected by an acceleration or gravitational force change, the beam will deflect around the fixed end. The beam is preferably wide, which would prevent the beam from twisting or deflecting in other directions than intended. The beam also can be tilted. This tilt and the width of the beam will produce sensitivity to acceleration and gravitation changes in a direction perpendicular to the mounting surface only. Thus, the piezoelectric accelerometer can be said to be of a monoaxial type. The width of the beam also enhances the magnitude of the current generated by the piezoelectric layer. When the accelerometer is subjected to acceleration and/or gravitational forces directed perpendicular to the mounting surface, the beam will deflect around the fixed end, and the piezoelectric material will generate charges in dependence of the rate and magnitude of the acceleration and/or gravitational changes.
Furthermore, according to this embodiment of the invention, the piezoelectric accelerometer is positioned in such a way within a pacemaker that, when the pacemaker is implanted in a patient, the accelerometer beam is positioned vertically with its direction of sensitivity being the anterior-posterior direction of the patient, with the advantages described above. Since the piezoelectric accelerometer is capable of providing negative values, the prone position can easily be distinguished from the supine position.
As indicated above, the invention also is applicable to other piezoelectric sensors, such as endocardial pressure sensors for measuring the intracardiac pressure.
It is for instance possible to determine changes in posture by means of an intracardiac pressure sensor. The hydrostatic pressure acting on the sensor increases when the patient rises from a prone or supine position to an upright position since the vertical distance upwardly from the sensor within the patient that defines the hydrostatic pressure will increase. The effects of an increase in pressure on the pressure sensor will generally be similar to the effects of acceleration or gravity on an accelerometer of the type described above. The arrangement described above used for evaluating the accelerometer signal thus could be used also for evaluating the signal from the pressure sensor. Since a pacer system normally contains some kind of activity sensor, the pressure signal also additionally could be evaluated by means of the signal from the activity sensor in order to better distinguish the rise in pressure resulting from a change in posture from a change in pressure resulting from a change in activity.
A further use of the arrangement according to the invention is to detect long-term changes or drift in the intracardial pressure by means of a pressure sensor.